Securing element

ABSTRACT

The invention relates to a screw-type securing element, made of titanium, for permanent anchoring in bone tissue, in particular for permanent anchoring of artificial teeth and dental bridges in the jaw bone, the securing element having an outer threaded portion which at the far top merges into a smooth, conical and/or cylindrical flange (5, 9). The thread end is such that the thread (1) in the periosteal part of the securing element merges into the flange (5, 9) via a cylindrical groove (7) which has the geometry of the threading die. By means of maintaining, in this way, a correct thread pitch and thread depth right up to the flange, a geometrically satisfactory match is obtained, in which the previously threaded hole matches the geometry of the thread right up to the flange, which means that the cortical bone can be better utilized.

FIELD OF THE INVENTION

The present invention relates to a screw-type securing element, made oftitanium, for permanent anchoring in bone tissue, in particular forpermanent anchoring of artificial teeth and dental bridges in thejawbone. The securing element has an outer threaded portion which at itsfar top merges into a smooth conical and/or cylindrical portion. Thefront part of the screw is preferably provided with one or more recesseswhose margins adjoining the circular symmetrical surface of the securingelement form cutting edges permitting self-tapping when the element isbeing screwed into the bone tissue.

BACKGROUND OF THE INVENTION

Screw-type securing elements, made of titanium, for replacing lost teethhave been shown to have many advantages. The outer thread of thesecuring element provides a natural positive locking in the bone andgives initial stability, and also distributes the load favorably to thesurrounding bone tissue. Recent long-term clinical follow-up studieshave underlined the fact that threaded securing elements are in thisrespect more advantageous than the unthreaded ones. The position of theboundary area where the thread merges into a smooth, cylindrical orconical surface is for this reason of great importance with regard tothe function of the securing element, i.e. the implant. This positionusually determines where, on the securing element, long-term stabilizingof the bone tissue occurs.

To obtain initial stabilizing of the securing element, and to place thelatter in a predetermined position, it is already known to have thethread end with a flange. The flange means that there is a possibilityof mechanical resistance with increased initial stability and results inmore reliable positioning. The initial stability is considered to beimportant for ensuring incorporation, and the improved positioning inthe axial direction affords greater protection against penetration intothe nerve channel of the mandible.

The flange also makes it possible for the soft tissue to seal off theoral cavity directly against the fixture, and any bacterial leakagethrough the spacer piece does not reach the bone level. The flange canthus be regarded as a first part of the spacer system attached to thefixture (or a first part of the implant part penetrating the softtissue).

However, when the thread is being chased, the flange represents anobstacle to the runout of the thread cutter, for which reason a turnedrecess is usually formed under the flange to lift the cutter out withoutdamaging the flange. However, this means that approximately one threadturn is lost on the securing element, which leads to the marginal boneheight lying correspondingly further down on the securing element. Thisloss is critical in some cases, since it means that it is not possibleto use the outermost bone edge, which normally has the best mechanicalproperties. It is also important, particularly in the case of thin bone,that the thread be fully used to obtain good initial stability of theimplanted securing element.

An alternative production procedure is to allow the thread cutter to bedrawn out only radially, but this results in an increasingly shallowerthread, which does not fit in the already threaded hole, and anundefined flange is obtained. Thus, this method also does not solve theproblem of how to fully use the outermost bone edge.

The loss of marginal bone height is especially critical in the use of asmall number of implants in the molar areas of the jaw, since anunfavorable loading can occur in this region, especially in the case ofan individual molar. A considerable increase in the strength of theimplant can, of course, be obtained simply by increasing the dimensionsof the implant, but it is far from certain that the existing bone volumewill permit this. Omitting the flange, and allowing the thread to runright to the top, is not an optimal solution either, considering theadvantages which a flange affords, namely being a counterstay for theinitial tightening, and an active, sealing part of the area of theimplant passing through the soft tissue.

The hardness of the bone in the molar areas of the jaw can vary greatly.In some patients, only a very thin outer layer, the cortical bone, ishard, while the remaining inner bone, the so-called spongiosa, is verysoft. In these bone types, it is already known to use self-tappingfixtures, see for example SE 468,154.

The advantage of self-tapping fixtures is that the implantation of thefixture in the jaw bone is simplified. The normal procedure in factinvolves drilling a hole in the bone. In this connection, drills ofincreasing diameter are successively used until the diameter of the holecorresponds with the core diameter of the threaded fixture. A threadingtap is then used, to form the thread in which the implant is placed.When using a self-tapping fixture of the type which is described in theabovementioned patent, the implantation is performed without using athread tap. However, the use of self-tapping fixtures does not, initself, solve the problem of loss of marginal bone height.

SUMMARY OF THE INVENTION

The object of this invention is to provide an implant which is based ona presently known basic design and dimensions, but in which the boundaryarea where the thread merges into the smooth, cylindrical or conicalportion, i.e. the thread end, has been given a novel configuration, withthe intention of having the marginal bone height lie higher up on theimplant so that the cortical bone is better utilized. This is achievedby the fact that the thread in the periosteal part of the implant mergesinto (i.e. ends at) the smooth, cylindrical or conical portion via acylindrical groove which has the geometry of the threading die.

In a first embodiment, the smooth portion, against which the threadends, is formed by a cylindrical flange. As has been pointed out in theintroduction, the flange constitutes a counterstay for the initialtightening of the implant and permits positioning of the implant at aprecisely determined height. By having the thread now end against theflange via a cylindrical groove which has the profile of the thread, nothread turn is lost on the implant. Consequently the cortical bone canbe utilized right up to the flange.

In a second embodiment, the smooth portion against which the threadends, is formed by a conical portion. In this case too, the thread endsagainst the conical portion via a cylindrical groove which has theprofile of the thread, in direct connection with the conical portion,and the cortical bone can be utilized right up to the conical portion.The conical portion in some cases provides the additional advantage, ascompared with a cylindrical flange, that precise preparation of theposition of the conical implant flange is made possible by using aconical countersink. This will be described in greater detail below.

The invention will be described in greater detail below with referenceto the attached drawing, in which:

FIG. 1 shows a previously known implant (prior art);

FIG. 2 shows a first illustrative embodiment of an implant according tothe invention, with a cylindrical flange;

FIG. 3 shows a second embodiment according to the invention, in whichthe flange is conical;

FIG. 4 shows an example in which the flange is made up of a combinationof a conical and a cylindrical surface; and

FIG. 5 shows an enlarged view of the thread end.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 shows a self-tapping fixture according to the BRANEMARK SYSTEMwhich is marketed by Nobel Biocare AB. The fixture consists of anessentially cylindrical screw with an outer thread 1, which merges intoa wider cylindrical flange 2 via an unthreaded transition portion 3 witha so-called "shallowing-out" thread. The screw is intended to beintroduced into a hole already drilled in the jaw bone for permanentanchoring of artificial teeth and dental bridges. The screw has an upperhexagonal portion 4 intended to cooperate with a tool for implanting thescrew. The screw is preferably made of commercially pure titanium with asurface structure in accordance with SE-PS 79.02035-0. The screw isself-tapping by virtue of its lower part being provided with threerecesses 5 formed in the circular symmetrical surface of the screw. Therecesses are designed to form, in connection with the circularsymmetrical surface, cutting edges 6, and together they have such avolume that the bone slivers cut off by the cutting edges areaccommodated within the recesses; see also above-mentioned SE-PS91.02451-3.

As mentioned in the introduction, there are certain applications wherethis fixture does not give the best results, namely in the molar areasof the jaw where there is a relatively thin, hard cortical bone and aninner, porous soft bone. When the screw is implanted in this region, therelatively elongate transition portion 3 with the shallowing-out threadcomes to lie opposite the hard cortical bone, and thus the advantages ofthe thread, with regard to positive locking and stability, are notexploited to the fullest extent.

FIGS. 2 and 3 show how it is possible according to the present inventionto minimize transition portion 3 by allowing the thread to end againstthe flange via a cylindrical groove 7 which has the profile of thethread at the point of connection to the flange. In this way, no threadturn is lost on the fixture, so that the cortical bone can be fullyutilized right up to the flange.

The thread end is produced in principle in two stages. When the threadcutter reaches the flange during production, the cutter is drawnradially outwards at an angle which, with respect to the axis of thescrew, is at least as great as the angle of the thread flank. In thisway, a correct thread pitch is obtained right up to the flange, and thethread merges into the flange with a shallowing-out portion. In thesecond stage in the production of the thread end, that part of theflange which adjoins the thread is then chased with a profile whichcorresponds to the profile of the thread. In this way, the thread mergesinto a groove 7 having the profile of the thread, which grooveconstitutes the connection of the flange to the thread, with graduallydecreased thread width and height, but with the thread depth beingmaintained. The thread runs right up to the flange, and at the same timethe flange remains completely intact, and the shallowing-out thread hasbeen eliminated. This also means that no part of the hole alreadythreaded in the bone is destroyed when the securing element isimplanted. With the previously known, shallowing-out thread, there was apossible risk of asymmetrical clamping effects, which can thus beavoided with the novel thread end.

One effect of the cylindrical groove under the flange is that there isno full contact between the flange and the uppermost thread turn in thebone. However, this disadvantage is considered small compared to thedisadvantage of having a thread which presses the bone asymmetricallyunder the flange.

The cylindrical flange 5 in FIG. 2 otherwise corresponds entirely to theflange on the previously known fixture, which is shown in FIG. 1. Thepitch and cross-section of the thread are also the same as present daystandards. Because a correct thread pitch and thread depth aremaintained right up to the flange, the profile angle of the flank 8,which is that part of the groove 7 adjoining the flange 5, approaches insize the flank angle of the thread, which in this case is 60°.

In FIG. 3, the invention is illustrated with respect to a conical flange9. In this case too, the thread adjoins the conical portion 9 via acylindrical groove 7, the flank 10 of the groove against the threadhaving the same profile angle as the flank angle of the thread, whilethe "flank" of the groove against the flange 9 in this case consists ofthe flange itself.

Fixtures with conical flanges are already known; see, for example,Swedish design registration 38.454. What is new in this case is thethread ending against the conical flange in the form of the cylindricalgroove 7. In addition, the conical flange has the same height as thecylindrical flange, i.e. a comparatively short flange in relation to theconical flange which is shown in the aforementioned design. In thepresent case, the flange has a height of 1.1 mm, its diameter at thethread connection is the same as the core diameter of the thread, i.e.4.9 mm, and its diameter at the upper plane 11 is 5.2 mm. The diameterof the fixture, the outer thread, is nominally 5.0 mm.

The reason why it is desirable, in certain applications, to work with aconical flange is that the latter, in conjunction with a conicalcountersink, permits a more exact anchoring of the implant. By providingthe already drilled hole with conical countersinking, which correspondsto the conical flange, a more exact adaptation between the cortical boneand the implant is obtained. The flange also helps in this way, togetherwith the thread, to provide the desired initial stability, which isespecially important in the comparatively thin cortical bone.

Upon application in areas with thin cortical bone and a soft trabecularcore, it is sometimes desirable to completely eliminate thecountersinking completely. Even in such a situation, the conical spaceris considered to be advantageous since it provides a graduallyincreasing resistance, which gives better protection against the flangelosing its hold than is possible with a cylindrical flange. In this typeof bone, it is often desired to place the fixture in a previouslydrilled seat of under-dimensioned diameter in the trabecular bone, whenit is desired to widen the entrance in the cortical bone. The conicalcountersink here makes it possible to widen the cortical bone carefullyin this area. The conical countersink can be used for holes measuringfrom 3 mm upwards. An advantage of the conical flange geometry is thatthe same countersink can be used for different fixture diameters. Thesurgeon can choose which degree of clamping effect is desired uponfitting each fixture, since the conical flange presses the bone outessentially radially. As long as the flange is not drawn under the boneedge, the stability will be maintained.

The conical countersink can also be used for widening the corticalentrance, instead of using a twist drill. The advantage of this is that,in such a reaming operation, the bone is worked radially, i.e.essentially in the plane of the bone, which means that it is possible toshape the hole to the desired diameter with considerably less risk offracture of the cortical plate. When a twist drill is used, the work isperformed essentially axially, which involves pressing on the boneshell. In addition, the twist drill has a tendency to cut agroove-shaped hole in thin plates, in which case there is a risk offragmentation.

The thread end for the conical spacer is also produced in principle intwo stages as discussed above.

In FIG. 4, the invention is illustrated with respect to a flange whichhas a lower conical part 12 and an upper, narrower cylindrical part 13.In this case too, the cylindrical groove 7 has a flank 10 against thethread, the flank having the same flank angle as the flank angle of thethread, i.e. 60°. The cylindrical groove 7 merges directly into theconical flange 12, which in this case has a cone angle of 23°. This isshown on an enlarged scale in FIG. 5a.

FIG. 5b shows an enlarged view of the case in which the flank 14, of thecylindrical groove 7 which adjoining the flange also has the geometry ofthe threading die, i.e. a flank angle of 60°. The case with acylindrical flange 15 and the case with a conical flange 16 have bothbeen indicated in the figure.

The invention is not limited to the embodiments shown by way of example,but instead can be varied within the scope of the patent claimsattached.

We claim:
 1. A screw-type securing element, made of titanium, forpermanent anchoring in bone tissue, the securing element having:a lower,threaded outer portion and an upper smooth outer portion, said threadedouter portion merging into said smooth outer portion via a cylindricalgroove, said cylindrical groove having a profile corresponding to thatof the threads of the threaded portion at a point of connection to thesmooth outer portion substantially along the periphery of the securingelement.
 2. A securing element according to claim 1 wherein the smoothouter portion, against which the threaded portion ends, is formed by acylindrical flange, and wherein a part of said cylindrical grooveadjoining the cylindrical flange comprises a flank whose profile angleis of the same size as the flank angle of the threads.
 3. A securingelement according to claim 1 wherein the smooth outer portion, againstwhich the threaded portion ends, is formed as a conical flange, andwherein the flank of the cylindrical groove adjoining the threads of thethreaded portion has the same profile angle as the flank angle of thethreads, and wherein the flank of the cylindrical groove against theconical flange comprises at least partly the conical surface.
 4. Asecuring element according to claim 3 wherein the diameter of saidconical flange at said point of connection is equal in size to the corediameter of the threads, and the diameter at the upper plane of saidconical flange slightly exceeds the outer diameter of the threads.
 5. Asecuring element according to claim 1 further including a forward partprovided with at least one recess whose margin adjoining a circularsymmetrical surface of the securing element forms cutting edges in orderto permit self-tapping when the element is being screwed into the bonetissue.
 6. A method for producing a securing element for anchoring inbone tissue and including a lower, threaded outer portion and an uppersmooth outer portion forming a flange, the threaded outer portionmerging into said smooth outer portion via a cylindrical groove, thecylindrical groove having the profile corresponding to that of thethreads of the threaded portion at a point of connection to the smoothouter portion, said method including the steps of:reaching said flangewith a thread cutter; drawing the cutter straight out at an angle which,with respect to the longitudinal axis of the securing element, is atleast as great as the angle of the threads flank; and chasing the partof the flange adjoining the threads with the profile of the threads andproviding said cylindrical groove with a profile corresponding to thatof the threads of the threaded portion at a point of connection to thesmooth outer portion substantially along the periphery of the securingelement.